One of the
most frequently asked questions is about the scales on Weston meters and
the Weston film speed, and how to tell if the meter is accurate.

With modern
films and cameras, setting the film speed is something you don't have
to worry about - you just buy an ISO 400 film for general use, or perhaps
ISO 200 or even 100. The speed is encoded onto the film canister, the
camera picks it up, and all the rest is automatic. Back in the 1940s,
film manufacturers were optimistic in varing degrees about their film
and their published speeds were just not trustworthy. Weston were making
very accurate exposure meters and they didn't want to jeopardise things
by relying on inaccurate speed ratings. So, they came up with their own
and published
their Weston Speed for all the popular films of the day. How do the Weston
speeds translate to todays ISO or even DIN ratings?

Then we have
the numbers on the Weston dials - what do they mean?

Firstly the
dials.

Up to the
Master IV the scale on the dial is in cd/ft2 - foot candles, now known
as Lumens. After the IV, Weston started to use arbitrary numbers. With
the baffle open, the scale is from 0.2 to 50 and with the baffle closed,
25 - 1600. Note the overlap - it's usually best to use the baffle-open
in this overlapping area as the meter is more accurate there.

Now the Film
Speeds.

Meters up
to and including the Weston III used the Weston Speed rating. From then
on they used ASA which is now better known as ISO. DIN was a European
rating until recently and GE was General Electric's own system used for
their meters.

DIN

ASA/ISO

GE

Weston

10

6

8

5

11

8

10

6

12

10

12

8

13

12

16

10

14

16

20

12

15

20

24

16

16

24

32

20

17

32

40

24

18

40

48

32

19

50

64

40

20

64

80

50

21

80

100

64

22

100

125

80

23

125

150

100

24

160

200

125

25

200

250

150

26

250

800

200

27

820

400

250

How to check
if the meter is reasonably accurate.

Make sure
that the pointer is reading zero with the cell covered. If not, adjust
with the screw on the front. Point the meter to a clear north sky at any
time between 10 A.M and 2 P.M. There must not be any clouds or haze. An
accurate meter will give a reading of approximately 320 Lumens - the scale
used before the Euromaster series. That's just over half way across the
dial with the baffle closed. This should translate to f/16 at 1/100, with
film speed set to ASA/ISO 100.